document.querySelector('#upload').addEventListener('change', function () {
  console.log(this.files[0]);
  const fd = new FormData()
  fd.append('img', this.files[0])
  axios.post('http://hmajax.itheima.net/api/uploadimg', fd).then(res => {
    console.log(res, 8);
    document.body.style.backgroundImage = `url(${res.data.data.url})`
  })
})